Have a dual wielding Rogue that is massacring the Stronghold.
Just found a Ring of Combat (+3, +7) and when I equipped it I saw that it only raised the To-Hit and To-Dmg of the wielding hand. ie. Ring in the right ring slot only affects the Main-hand weapon. Moved the ring to the left ring slot and it now applies to the off-hand weapon.
Is that working as intended?

Not sure if the Rogue will make a good dual wielder in the long run, but at least early it seems impressive. Keeping a shield to swap in when I hit tougher mobs.

I was surprised by this just like you, when someone mentioned that I should put my ring of combat on the weapon hand to get the to-hit/damage bonusses.
This is apparently working as intended. It is probably to avoid extreme benefits from dual wielding and a single great ring of combat to offset the dual wield penalties.
I also just now learned that you get bonus damage from a single weapon when not wearing a shield.

I was surprised by this just like you, when someone mentioned that I should put my ring of combat on the weapon hand to get the to-hit/damage bonuses.
This is apparently working as intended. It is probably to avoid extreme benefits from dual wielding and a single great ring of combat to offset the dual wield penalties.
I also just now learned that you get bonus damage from a single weapon when not wearing a shield.

I think the two-hand benefit applies to larger weapons, not Daggers and Tantos, but hey, I haven't played *Hengbands since like 2005 so what do I know?
I have a kick-ass Wakizashi that I have trained to useable. Using Craft Magic on my Rogue, massive early game bonus to counteract Dual-wield penalties. Need the stores to restock !CCW and I will be good to go, cure Serious aren't even worth carrying for HP.

Odd thing with Dual-wield. The character dump has odd artifacts, Wielding two identical Daggers (+5, +5) and the Weapons reports give different to-hit, crit rate and Accuracy.